The tread of a modern tire extends around the outer circumference of the tire and is designed to contact the road surface. The tread is typically divided into a plurality of raised, load-supporting tread blocks separated by intersecting circumferential and transverse grooves and/or sipes. The tread blocks determine the control, acceleration and braking characteristics of the tire, while the grooves and/or sipes are necessary to provide flexibility and water removal.
The tread blocks are typically arranged in "base pitches" around the tire, wherein each base pitch includes a predetermined geometry of whole and/or partial tread blocks separated by grooves and/or sipes. Each tread block in the base pitch can be separated from the other tread blocks by circumferential grooves, transverse grooves, and/or grooves which extend in other selected directions. Likewise, the tread blocks can be separated or interrupted by circumferential sipes, transverse sipes, and/or sipes which extend in other selected directions, as is known to those skilled in the art. In other words, each base pitch has a particular tread geometry which is chosen by the designer at least in part according to the factors outlined above.
Additionally, each base pitch may vary in width across the tire. For example, each base pitch may extend transversely from one shoulder to the other shoulder of the tire. Alternatively, there may be several rows or ribs of base pitches, each of which extend transversely over only a single circumferential section of the tire, for example over a single rib or combination of ribs. In any case, the base pitches are repeated around the circumference of the tire to form the complete tire tread.
The constant contact of the tire tread with the road surface during use can often create unwanted noise. In particular, as the tire contacts the road surface, the individual tread blocks cause air disturbances upon impact with the road, creating a spectrum of audio frequencies broadly referred to as "tire noise". Tire noise is generated at least in part by: (1) the impact of the tread block on the road surface; (2) the vibration of the tire carcass; and (3) the "air pumping" which occurs as the tread blocks become compressed and expand into the grooves separating adjacent blocks.
Various tread designs have been developed which attempt to reduce the noise of the tread on the road surface. Such noise treatment techniques attempt to distribute the noise frequency produced by the tire tread over a wide frequency band to approach what is termed "white noise". For example, one known technique for reducing tread noise is to use base pitches having different pitch lengths, wherein the "pitch length" is a measure of the length from the leading edge of one base pitch to the leading edge of the next adjacent base pitch in the circumferential direction of the tire. A plurality of base pitches having different pitch lengths is conventionally referred to as a "pitch sequence".
Lippman et al U.S. Pat. No. 2,878,852, discloses a tire tread having male and female mold halves, wherein each mold half has a separate pitch sequence extending around the circumference of the tire. Lippman discloses a pitch sequence which is represented by the series of relative circumferential distances: 9 10 11 12 10 11 12 13 12 11 10 9 11 12 13 10 10 13 12 11 9 10 11 12 13 12 11 10 12 11 10 9. These circumferential distances represent the relative circumferential length of successive tread units expressed in any desired measuring unit. In Lippman the tread units are selected to be mirror images of themselves or of other groups in each respective mold half.
Another technique for reducing tire tread noise is shown in Williams, Ser. No. 07/792,006, filed Nov. 14, 1991, entitled "Tire Pitch Sequencing Technique", which is owned by the assignee of the present invention. Williams specifies certain pitch length criteria for arranging the base pitches relative to each other in the pitch sequence to reduce the noise of the tire.
Other techniques use random or sinusoidal sequencing of the pitches in an attempt to modulate the objectionable noise producing frequencies and spread the tire noise over a broad frequency spectrum (see e.g., Vorih, U.S. Pat. No. 3,926,238).
It is noted that the Lippman, Williams and Vorih disclose varying the pitch length of each base pitch according to certain criteria. However, in all other respects, the tread geometry of each base pitch is identical to the tread geometry in an adjacent base pitch in the pitch sequence. In fact, typical tire tread patterns have a uniform tread geometry used for each base pitch which is repeated around the entire circumference of the tire using techniques such as described above. All of the base pitches of the tread pattern appear similar, except for their variations in circumferential pitch length.
Accordingly, the above references disclose certain techniques which are designed to reduce noise produced by the tire tread when contacting the road surface. However, there is a constant demand in the industry for new and improved techniques for providing a tire with reduced tire tread noise, and in particular, techniques which can be used in addition to, or alternatively to, the techniques described above for providing a tire which has a reduction in tire tread noise on the road surface.